Refractory molds



Nov. 17,1959 J. M. WEBB REFRACTORY MOLDS Filed July 24, 1956 INVENTOR John M Webb zw zzom,

ATTO NEYS III Ill-l l- United States Patent 2,912,729 REFRACTORY MOLDS John M. Webb, Chagrin Falls, Ohio Application my 24, 1956, Serial No. 599,779 'S'Claims. (01. 22-129 The present invention relates to an improved refractory mold for precision casting of metals having high melting points and to a method of making such molds.

Small parts made of metals having high melting points, such as titanium, stainless steel, etc., are cast into thinwalled refractory molds backed or supported by sand. The molds are generally formed by dipping a frozen mercury pattern into slurries of refractory material as is described jin;U.S. Patents Nos. 2,400,831, 2,682,692 and 2,749,586.v

The-refractory molds must satisfy exacting requirements. The walls, of the mold must have high physical and thermal shock properties so that they do not crack when metals such as molten titanium, stainless steel etc. at temperatures of around 3000 F. are poured into them. The walls of mold must also be quite thin so as to yield and not crack when the cast metal contracts about the mold walls during cooling and solidification.

Up to the time of the present invention, the only satisfactorycasting method was to support the thin-walled molds in sand or other investing materialduring the pouring operation. The sand had to be packed around the thin-walled mold by vibration or other techniques known in the art and then, after the metal had been cast,

the sand had to be separated out, and cleaned before it was reused.

Also, the mold and sand or other investing material had to be heated over a relatively long period of time to a temperature of 850 up to 1500 F. or more so that the molten metal which was being pouredinto the mold would flow through the mold passageways. When a mold with a small crack was so heated, the heating tended to open the crack to a larger extent and ruin th: mold.

An additional limitation on precision casting processes is the fact that contamination of the molten metal by the mold must be avoided. The problem is especially severe with active metals such as titanium.

The present invention relates to a laminated, multiwalled'm'old of refractory materials which comprises (a) an inner portion comprising a thin inner prime layer which has a smooth, hard, non-contaminating casting surface and a thicker second concrete layer which supports the first layer, (b) an intermediate buffer portion comprising a layer of porous refractory material having relatively'low compressive strength and formed from a composition without a high temperature binder, and (c) an outer portion comprising a layer of refractory material having relatively coarse particles as a major part of its refractory ingredient and a relatively high percentage of a high temperature binder. Using the mold described above, I am able to cast metals such as stainless steel and titanium without sand backing, without contamina- 2,912,729 Patented Nov. 17, 1959- tion of the metal, and with a minimum of mold loss and other processing.

One object of the present invention is to provide an improved refractory mold which does not crack when hot molten metal is poured into the mold. v

Another object of the present invention is to provide an improved casting mold which can be sealed by heating if it should happen to have become cracked during firing. i

Still another object of the present invention is to provide a mold in which titanium, stainless steels and the like may be poured without the need for a backing such as sand or other investing material.

Other objects, uses and advantages of the present invention will become apparent to those skilled in the art from the following description and claims and from the drawings in which:

Figure 1 is a perspective view of a vane of a gas turbine to be made in accordance with this invention;

Figure 2 is a fragmentary elevational view of a frozen mercury pattern for the vane with parts broken away and shown in cross section;

, Figure 3 is a fragmentary elevational view of the frozen mercury pattern with a plurality of shell mold layers formed thereon; and

Figure 4 is a sectional view taken along the lines indicated at 44 in Fig. 3 and on an enlarged'scale.

In the present invention, the mold is preferably formed on a frozen mercury pattern and the discussion will be directed to this casting process. It should be understood, however, that the invention is equally suited to other casting processes, such as the lost wax process, provided the pattern material can be removed from the mold'without damaging it.

Referring now particularly to the drawings, Figure 1 shows a vane 5 of a gas turbine engine which is represen tative of the type of parts cast in molds of the present invention. Vane 5 has thin concave blade section 6 and a thin convex back: section 7 which are joined along a front edge section 8 and a rear-edge section 9 so as to form a hollowinterior 10.

Figure 2 shows a frozen mercury pattern 11 of gas turbine vane 5 with a sprue 15 of frozen mercury attached thereto by means of linking sections 16 of frozen mercury. This linking provides passages to the interior surfaces of the hollow frozen mercury pattern 11 which interior surfaces will be adjacent to the hollow interior 10 of the finished casting. A metal hook 17 is frozen in the sprue position 15 for handling the pattern 11 during the mold forming process.

In accordance with the present invention, a frozen mercury pattern 11 is prepared by pouring the mercury into a metal alloy mold not attacked by mercury, freezmg the mercury, and removing the solid frozen pattern 5. The frozen mercury pattern 11 is then alternately dipped into a series of slurries of refractory particles and dried as will be more fully described herein to form a thin laminated shell mold 20 comprising an inner portion, an outer reinforcing portion, and an intermediate buffer portion, to equalize or absorb strains between the inner and outer portions.

The inner portion comprises two or more layers of refractory material. The first or prime layer 26 essentially comprises fine refractory material and suitable binder as hereinafter described. A fine refractory material is de'-' sired so as to present a smooth surface to the molten metal and form a smooth surface on the casting. It should be free of contaminating elements such as phosphorous and materials which would react with the molten metal or form gases which may be occluded in the molten metal. The second or concrete layer 27 supports and reinforces the first layer and is made of a stronger and less fine material. Except in unusual circumstances there is no contamination problem and the second layer may con- 'tain ingredients which are superior to those in the first layer except with regard to contamination For example, as hereafter explained in the discussion of binders, the second layer may not have ammonium phosphate as the high temperature binder. Ammonium phosphate is an excellent high temperature binder because it fluxes and binds the refractory materials together at from 650800 F. but it cannot be used in the first layer because phosphorous contaminates the metal. Additional reinforcing or prime layers of refractory material may be used in the inner portion as desired, but in most cases are not necessary.

The prime layer should have a thickness of from 20-60 percent of the thickness of the inner portion and preferably should have a thickness of 30-50 percent of the thickness of the inner portion. The second or concrete layer is preferably thicker than the first or prime layer and may be up to four times as thick.

The intermediate bufier portion is formed by a third or buffer layer 28 applied over the second layer 27 of the inner portion. The intermediate buffer portion comprises one or more layers of relatively weak, crumbly, porous refractory material. A high temperature binder is preferably not used in the initial refractory mixture because it reduces porosity and supplies strength. Porosity and the ability in the bulfer portion to permit adjustment of movement between inner and outer portions as the temperature changes are desired. The refractory material preferably comprises a major part of zirconium silicate and a minor part of aluminum silicate and the refractory ma- :terial preferably should comprise at least 95 percent of the initial mixture with the balance being low temperature binder. The refractory material also preferably comprises a major part of particles that are of a mesh size of 150 to 400 mesh and a minor part of particles that are of a mesh size of 35 to 150 mesh.

Even though the buffer layer is at least two layers away from the prime layer, it absorbs and minimizes the shocks and strains on the prime layer which occur when the molten metal is cast into the mold and when the metal cools and solidifies. The buffer layer also absorbs and cushions expansion and contraction strains set up between the various layers as they first expand and then contract when the metal is cast and solidifies. At the same time, it strengthens the mold so that sand backing or other support is not necessary.

Surrounding the intermediate buffer portion is a coarse outer portion comprising at least one layer 29 which supports and backs the interior portions. The refractory material of this portion should be from 40 to 85 or 90 percent by weight of relatively coarse particles such as zirconium silicate of a mesh size between 35 mesh and 150 mesh with a minor portion of finely divided particles such as zirconium silicate between 150 and 400 mesh. A very high temperature binder which may be difierent from that in the inner layers is also present in the outer portion as herein described.

In order to obtain certain castings, it may be desirable to build up the thickness of the third buffer layer 28 and/or the fourth layer 29 in some sections of the mold in which additional stresses are expected or where a slower rate of cooling of the metal is desired.

The total thickness of the mold 24 should be generally about A to of an inch. If the first two layers are about A inch or less thick, the third and fourth outer layers generally should be slightly thicker, say about inch thick.

The above dimensions may be used for a mold for a casting weighing ten pounds, which is a medium sized casting. The mold should be thicker for heavier castings, up to /2 inch thick, and may be thinner for smaller castings although the mold is usually not thinner than inch thick.

The inner portion should preferably comprise from 20 to 40 percent of the total thickness of the mold. The total thickness of the inner portion for a medium sized casting should be from to of an inch. When the inner portion comprises two layers with the thickness of the prime layer about 40% of the inner portion, the buffer layer is preferably about as thick as the prime layer. The outer layer is preferably about as thick as the concrete layer, which is also a hard strong layer.

The mold layers are built up by dipping the pattern in a slurry, removing the pattern to allow the liquid slurry medium or dispersant to evaporate and deposit a refractory layer on the pattern, and then again dipping the pattern in the slurry. The cycle usually has to be repeated three or more times for each layer of refractory material deposited on the pattern.

The pattern and built-up refractory layers are then heated or fired to liquefy and drain off the pattern material and form the final refractory mold as is well known in the art.

Prior to firing the mold is held together by low temperature binders comprising organic resins and the like. The molds are subsequently fired or heated to a tempera ture sufiiciently high to cause the high temperature binder to bind refractory particles together and also high enough to decompose or modify the low temperature binder so as to provide gases which render the mold porous. Hot molten metal is then cast into the hard porous mold to form an improved precision casting of which the vane 5 is representative. Since mercury freezes at about -40 F., the liquid carrier for the slurry of the refractory compositions using the frozen mercury process must be a liquid at below about -'40 F., and have a high enough boiling point so that it evaporates before the mercury becomes liquid. The boiling point of the liquid carrier is preferably below about 70 F. and the liquid carrier is preferably one that is a gas at room temperature, such as monochlorodifluoromethane or Freon 22 manufactured by E. I. du Pont de Nemours and Co., Wilmington, Delaware. Other suitable liquid carriers include liquefied methyl chloride, difiuorodichloromethane, trichloromonofiuoromethane, or a mixture of two or more such liquids. Liquefied monochlorodifiuoromethane, methyl chloride and dichloromonofluoromethane or a mixture of two or more such liquids may also if desired operate as solvents for some of the low temperature binders such as polymerized butyl methacrylate, and polymerized isobutyl methacrylate and polyvinyl acetate.

The liquid carrier should be used in large enough amounts to provide a slurry of a desired viscosity which is generally about to 300 centipoises at 76 F. The thickness of the slurry is important since generally, if the slurry is thick, the coating will be thick. Also, if the slurry is too thick, it will take an excessive time to evaporate the liquefied carrier. If the slurry is too dilute, the pick-up on each dip will be insufficient from the standpoint of economical operation.

Other liquid carriers or dispersants having higher boiling points may, of course, be used when the pattern is wax as used in the lost wax process or a low melting metal such as Woods metal. For example, water, carbon tetrachloride, light oils, and many other dispersants may be used.

Inasmuch as the mold is formed by dipping from an aqueous slurry at sub-normal temperatures and inasmuch as the mold must maintain its shape and have substantial strength when the mercury is removed as Well as at temperatures of 2000 to 3000" F. or so, it is generally necessary that the refractory material be bound together by a plurality of materials which are operable to provide strength at different temperatures. Binders are ordinarily a glue-like material which becomes effective only after solidifying from the liquid or viscous state, or by reacting with other materials present to form a solid mass. Binders which are effective at low temperature are or dinarily organic resinous materials or solutions of materials organic or inorganic which harden upon evaporation of solvent but decompose or burn at high temperatures. Examples of the binders which are effective at low temperatures include polymerized vinyl acetate, polyvinyl alcohol, ethyl cellulose ethers and esters, epon resins, polymerized acrylic esters such as polyethyl methacrylate, normal-butyl methacrylate, rubbery polymers particularly polar rubbers such as nitrile rubbers formed from 30-70 percent by weight acrylonitrile and 70-30 percent butadiene, and other rubbery polymers soluble or dispersible in the mold forming slurry. Most of the low temperature binders and those ordinarily preferred are combustible materials which are removed by heating the mold at high temperatures without a permanent reaction solidifying the powdery materials.

The low temperature binders also provide a means for controlling porosity of the mold. Increased amounts provide increased porosity after it is removed.

Priortothe temperature where the low temperature binder is removed, a binder effective at higher temperatures but also somewhat effective before the effect of the low temperature binder disappears, is generally desirable, at least in the layers adjacent the intermediate buffer portion.

Generally the low temperature binders lose most of their effectiveness at 600 or 700 F. so that it is desirable toprovide in each two successive contacting layers, a binder which is effective atthis temperature. One of the best of the intermediate temperature binders which we have found is mono basic ammonium phosphate which is effective from 500 or 600 to 1100 or 1200 F. When low temperature slurries are not required, one may also use in place of or in addition to mono ammonium phosphate, other phosphate compounds such as phosphoric acid and sodium phosphate and other alkali metal phosphates. Also, when slurries are not used at low temperatures, solutions of alkali metal silicates 'and the like are also effective as intermediate binders. However, I have found ammonium phosphate to be the most advantageous of the intermediate binding materials of which I am now aware.

As before mentioned, the phosphates are not desirable in contact with the metal casting because of contamination thereof and are therefore undesirable in the prime coating of the casting. Phosphate intermediate binders are, therefore, preferably not used in the prime coating and are used in the concrete coating which supports the prime coating in this period.

The fluoride compounds, such as, for example, alkali metal fluorides including sodium and ammonium fluorides, and also alkaline earth metal fluorides are excellent binding materials which become effective upon heating to extremely high temperatures and which remain operable at such temperatures. Alkali metal fluoride compounds are also advantageous in that they do not contaminate the casting. In order to make them fuse or react and become effective at somewhat lower temperatures combinations of one or more fluorides and one or more boron compounds such, for example, as boric acid are used in some of the layers of the mold, preferably the prime layer. The concrete layer of the inner portion, which directly reinforces and supports the prime layer during the intermediate heating portion may or may not contain a fluoride or boron compound and such is preferably not present therein, because there is sometimes a tendency for the fluoride and phosphate materials when present in substantial, proportionsin the same layer,,to

react and migrate into the prime-layer and thus'cause phosphate contamination of the casting. Also, the combination of phosphate and fluoride weakens the structure during the intermediate period after firing. The combination. of an alkali metal fluoride and a boron compound start to become effective in the range of 800 to 1100 or 1200 F. The alkali metal fluoride alone becomes effective or reacts at 1100 F. or higher. Thus the layers containing the high temperature binders are supported by layers containing intermediate temperature binders at temperatures of 300 or' 400 up to 800 or 1100 F.

For highest strength the absence of a boron compound is desirable. Therefore, in the coarse outer layer where strength at pouring temperature is the prime requisite the substantial absence of boron compound is preferred. Also, presence of the boron compound in substantial amounts while decreasing incipient effective temperature also decreases the high temperature strength obtained with an alkali metal fluoride binder alone.

As hereinafter pointed out I have also found'thatvthe use of the combination of Vycor glass and fluoride-boron containing binder results in a material which becomes strong at intermediate temperatures so that the presence of intermediate binder such .as the aforementioned phosphates, is not required. Apparently the Vycor type glass andboron-fluoride compound results in a partial eutectic being formed which causes the extremely high temperature binder to become effective at lower temperatures of about 500 to 600 F. or so.

The amount of low temperature binder in the prime layer is preferably about 0.1 to 5% by weight of the solids in the prime layer slurry. The prime layer also preferably contains from about 0.1 to 5% of the high temperature binder of which a mixture of sodium fluoride and boric acid is generally used.

The concrete layer preferably contains about 0.1 to 5% by weight of a low temperature binder which generally comprises polyvinyl acetate, ethyl cellulose and phenol formaldehyde resin as in the prime layer. The concrete layer preferably contains from about 0.1 to 5% by weight of an intermediate temperature binder such as an ammonium phosphate which becomes effective at intermediate high temperatures.

The bulfer layer preferably contains about 0.1 to 5% by weight of the low temperature binder such as'poly'vinyl acetate alone or a mixture of it with ethyl cellulose and phenol formaldehyde resin as in the prime and concrete layers. The buffer layer is preferably formed without a high temperature binder.

The coarse outer layer preferably contains from about 0.1 to 5% of a low temperature binder of which the same conciipositions as described in other layers are generally use 1 When the amount of high temperature binder in the outer reinforcing layer, i.e. the outer portion of the mold, comprises less than 2% of fluoride the strength is usually lnsuflicient to permit casting without support such as sand, shot, or the like. When the amount of fluoride high temperature binder is greater than-12% based on the total weight of solids thereof, the porosity is somewhat insuflicient to prevent proper formation of castings which are free. of gaseous entrapment at the surface thereof. Preferably the amount of high temperature binder is 4% to 8% of the total weight of solids. Examples of fluoride binders suitable for high temperature use particularly in the outer layer are alkali metal and alkali earth'metal fluorides such as sodium fluoride, lithium fluoride, potassium fluoride and calcium fluoride. Compounds such as cryolite, sodiumsilicofluoride, potassium silico fluoride and alkali fluoroborates including sodium and potassium fluoroborates which contain an alkali or 'alka'- line earth metal fluoride. Silicates and silica containing materials are not desirable in the inner portion especially in the prime layer when titanium is cast because of interaction.

Intermediate high temperature hinders such'as mono ammonium phosphate may also be present in the outer portion but are not generally desired because of decreased strength and porosity. If a binder operable at lower temperatures is desired, a boron compound may be present as above stated. Fluoride compounds are preferably used in the outer layer, however, in the absence of a boron compound to provide the correct strength needed for the pouring of metal in unsupported molds. The alkali metal fluorides such as sodium and potassium fluoride and particularly the sodium fluoride are the preferred fluoride compounds for high temperature binder use.

When in any layer a mixture of fluoride and boron compounds is used as the high temperature binder, the percentage of fluoride compound such as sodium fluoride is generally about 3 to 6 times by weight of the boron compound such, for example, as boric acid. The preferred relatively high percentage of fluoride aids the fluxing and binding of the mixture of coarse and fine particles by reaction between the fluoride and the refractory material.

The particle size of the binders is usually 325 or 400 mesh, and preferably less than 100 mesh. High temperature binders may also be incorporated in the mold after initial baking as shown by US. Patent 2,749,586.

A low temperature binder should be present in all slurries in amounts sufficient to bind the refractory particles together from the temperatures of the slurry up to the temperatures at which the intermediate or high temperature binder present in that slurry or in the slurry forming a supporting layer becomes effective as the permanent binder for the refractory particles. The low temperature binder should be capable of volatilizing, decomposing or otherwise being modified so as to form gases which in turn render the mold layers porous upon firing and permits releasing of gases through the mold walls.

The refractory materials of each layer vary with the purpose and function of the layer. Refractory materials for the inner casting layer 26 should be materials which are resistant to reaction with the hot molten metal and are capable of being finely ground or reduced in size. The resultant inner casting surface should be smooth and nonreactive with the hot molten metal. The preferable refractory material for the inner casting layer used for casting titanium is pure stabilized cubic zirconium oxide which has been stabilized by leaching away silicon dioxide by acid. Stabilized cubic zirconium oxide is also preferably used for alloys containing various percentages of tatanium such as those containing about two percent'or more of titanium. Refractory materials suitable for inner layer refractory materials for casting metals other than titanium are zirconite, mullite-kyanite, alumina and quartz. These materials may be used alone or in combinations. The mesh size of the refractory materials is generally about 100 to 400 mesh.

While the refractory material for the prime layer is preferably 150 to 400 mesh, the other layers, namely the concrete and the buffer layer, have preferably a minor part, up to 50% of their refractory material, of a mesh size from 10 or 12 to 150 mesh. However, in the outer layer, the major part of the refractory particles are of a mesh size of 12 or 35 up to 150. The use of relatively coarse particles provides strength to the layer and also allows the layers to be built up at a faster rate.

The particle size of the major part of the refractory materials for any of the portions or layers generally should be from about 100 to 400 mesh, and preferably is 140 mesh to 325 mesh, as measured by a standard screen having 325 openings per linear inch. One suitable retfractory material is Zirconite, a zirconium silicate (ZrO .SiO which is generally used in a mesh size of about 200 to 400. Another suitable refractory material, generally sold in a mesh size of 35 to 150 mesh, is a bulky Mullite Kyanite material which is a mixture of aluminum silicates, the formula for Mullite being 3Al2O .2SiO and the formula for Kyanitebeing Al O .SiO Generally other refractory materials resistant to high temperature such as quartz, alumina, silicon carbide, magnesium oxide and flint may be used alone or in combination with the refractory materials noted above.

The refractory material in all layers and portions is preferably present in amounts of at least 90% by solids weight of the composition of the inner casting layer prior to firing, the dispersant not being considered as part of the original composition. Generally the same range is preferred for the second layer 27 and the third layer 28 although good results may be obtained with percent refractory material in one or all of the layers.

At least about 5% of a powdered high silica glass is preferably used in the second layer 26 in order to control phosphorous migration. Best results are obtained with 15 to 25 percent high silica glass. High silica glass may even be the sole refractory material of the second mold layer 27. The second layer should be thicker and stronger than the first layer and may be compounded to include a high temperature binder such as ammonium phosphate as already noted or other components which cannot be used in the inner layer because of contamination of the metal.

The refractory material for the inner layers 26 and 27 for molds used to cast metals such as stellite type alloys, in which a mold capable of rescaling upon heating is desired, preferably should contain at least about 5 to 10 percent of the finely ground, high-silicon oxide glass. It is made by smelting ceramic materials to form a molten glass which is thereafter cooled to the solid state. About one-third of the weight of glass is chemically leached away and the remaining material is refired, cooled, dried and powdered. The material is supplied by Corning Glass Works, Corning, New York, as Vycor. This material comprises about 96 percent silica and a small quantity of boric acid (about 2.5 percent B 0 together with traces of aluminum, sodium, iron and arsenic. The high silica glass has a specific gravity just under 2.18 and a softening point of 2730 F. The glass has a vitreous rather than crystalline structure and a uniform thermal expansion.

The examples that follow are intended to illustrate the present invention and are not to limit it in any way.

EXAMPLE 1 Inner casting layer slurry Percentage of solids by weight High temperature binder:

Sodium fluoride 0.75

Boric acid 0.25 Low temperature binder:

Polyvinyl acetate 1.50

Ethyl cellulose 1.00

Phenol-formaldehyde resin 0.50

Refractory material: Stabilized cubic zirconium oxide 96.00 Dispersion medium: Freon 22.

The slurry was prepared by first thoroughly mixing the dry dense components of the solid composition in a double cone mixer and thereafter cooling the mixed batch and slurrying it in Freon 22 (monochlorodifluoromethane) at a temperature of 60 F. The pattern was Second layer slurry Percentage of solids by weight Dispersion medium (Freon 22"). High temperature binder: Ammonium phosphate (monobasic) 2 Low temperature binder:

Polyvinyl acetate ethyl cellulose 1.7

Phenol formaldehyde condensation product in its intermediate soluble stage. Refractory material:

fZirconite 6 Mullite-Kyanite material 36 A mixture of 3Al2O3.2SiO2 (mulllte) and Al .SiO2

(kyanlte) made by the Kyanite Mining Company, Collin, Virginia.

The second layer was formed to a thickness of about of an inch in the same manner as the first layer.

A third porous layer 35 inch thick was formed from a third layer slurry without a high temperature binder by dipping the pattern into a slurry composed as follows:

Third layer slurry Parts solids Ingredients: by weight Low temperature binder (polyvinyl acetate) 1.5 Zirconium silicate 62.0 Aluminum silicate 37.5

Dispersion medium (Freon 22).

The pattern was then dipped in a fourth layer slurry to form a fourth outer layer having coarse outer particles and a thickness of about inch. The slurry is composed as follows:

Fourth layer slurry The mercury was then liquefied and drained out so that the low temperature binders supported the green mold structure. The green mold was then fired at 1850" F. for about 30 minutes and a hard smooth, porous ceramic mold was obtained.

The fired mold was then placed in a vacuum chamber with a pressure of less than 1 mm. mercury and with an inert atmosphere of argon. Hot molten titanium metal at a temperature of about 3300" F. was then cast into the mold unsupported and unbacked. The casting was excellent and no surface defects were observed in the completed casting even though the titanium metal is extremely reactive in its molten state.

When a mold was prepared from the same slurries as those of Example 1 with the exception that zirconium silicate was substituted for the stabilized cubic zirconium oxide in the inner layer slurry, the resultant titanium casting was unsatisfactory, one of its defects being a porous spongy surface apparently from the reaction of the titanium metal and particles of the mold. Also, some gases apparently had been entrapped in the finished titanium casting.

EXAMPLE 2 A mold was prepared from the same slurries as those of Example 1 with the exception that a mixture of 80% zirconium silicate and 20% of a finely ground high silica glass, Vycor, was substituted for the zirconium silicate in the second layer slurry, and the zirconium oxide of the first layer slurry was replaced by zirconium silicate.

The mold layers of Example 2 wereformed as in Example 1 and the ceramic mold was further processed as in Example 1 so as to obtain a hard thin-walled mold. Since this mold was used for casting stainless steels, the use of a vacuum and an inert atmosphere was not necessary during the casting step.

Instead of going through the usual preheating treatment of heating a supported mold to a temperature of 1000 to 1600 F. for about 4 to 6 hours so as to heat both the mold and the backing material, the mold of this example was used in an unsupported manner to cast stainless steel. An excellent stainless steel casting similar to the vane of Fig. 1 was produced by heating the unbacked mold of this example in a gas-fired furnace at 1400" F. for only about 5 minutes and thereafter pouring the stainless steel into the mold through an opening .at the top of the furnace. Thus a good precision casting was prepared without the time and expense of a long preheat period and equipment involving the use of a vibrator and a suitable flask and investing material.

The use of a third buffer layer between a strong concrete layer and an additional fourth outer layer comprising a high content of high temperature binder and predominately relatively large coarse particles of a refractory material such as zirconium silicate produces a mold in which the thin veins may be heated quickly and con-, veniently to a temperature at which the molten metal will be able to flow throughout the mold before being chilled.

The mold of Example 2 is useful for casting metals such as stainless steels, stellite alloys, plain carbon steels and aluminum. Stellite type alloys are a group of hard, corrosion-resistant, non-ferrous alloys containing 40 to percent cobolt, 20 to 35 percent chromium, 0 to 25 percent tungsten, 0.75 to 2.5 percent carbon and 0 to 3 percent silicon.

Patterns were prepared from wax and Woods metal, and dipped into the slurries of Example 1 to form refractory molds. The molds were heated to drain oil" the pattern material and then baked. Stainless steel was cast into the molds. The resultant castings were good except for some loss of dimensions. No reaction of the stainless steel with the mold materials and no entrapment of gases in the cast metal was found.

When wax or Woods metal is-used for the solid material, the high temperature binder in the concrete layer may be phosphoric acid instead of ammonium phosphate since the pattern need not be coated at sub-zero temperatures and there is no danger of the phosphoric acid freezing.

When at least 5% by weight of the refractory ingredient of the first inner layer and the second layer of Example 2 is Vycor, a high silica glass in powdered form, any break that occurs in the mold before casting tends to close up upon preheating in preparation for the casting operation. The ordinary ceramic mold that is cracked will tend to develop an even wider crack upon additional heating. The high silica glass apparently aids the high temperature binder in the fluxing and bonding together of the refractory materials as well as providing a smooth hard inner surface.

When at least 5% by weight of the refractory ingredient of the second layer is a high silica glass, mono basic ammonium phosphate may be omitted from the second layer and yet a supporting layer with good strength is obtained. This eliminates the danger of contamination of the cast metal with phosphorous.

It is well understood that, in accordance with the provisions of the patent statutes, variations and modifications of the specific invention may be made without changing the spirit thereof.

What I claim is:

1. A fired refractory mold for casting molten metals with a wall thickness of less than one-half inch comprising (a) an inner portion having at least one inner prime layer which is about 20 to 60 percent of the thickill ness of the inner portion, said prime layer formed from relatively fine refractory materials bonded together with a high temperature binder and being free of metal contaminating materials and at least one concrete layer overlaying said prime layer and stronger than the prime layer so as to reinforce and strengthen said prime layer, said concrete layer formed from refractory particles that are coarser than the refractory particles of the prime layer, (b) an intermediate buffer portion having at least one layer formed from refractory materials lacking a high temperature binder and not bonded together so that the buffer portion is crumbly and weak in compressive strength and (c) an outer portion comprising at least one layer formed from a mixture of at least 40% relatively coarse refractory particles, relatively fine refractory particles and a high temperature binder consisting of an inorganic fluoride compound which bonds the refractory particles together.

2. A fired refractory mold for casting molten metals with a wall thickness of less than one-half inch comprising (a) an inner portion comprising about 20 to 60 percent of its thickness of at least one prime inner layer formed from relatively fine refractory materials bonded together by a high temperature binder and being free of metal contaminating materials and at least one concrete layer stronger than the prime layer and formed from refractory materials that are at least in part coarser than the refractory materials of the prime layer, said refractory particles of the concrete layer comprising at least high silica glass and said refractory particles bonded together with mono ammonium phosphate as the high temperature binder thereof, said concrete layer overlaying said prime layer so as to reinforce and strengthen said prime layer, (b) an intermediate buffer portion having at least one layer formed from a major part of zirconium silicate and a minor part of aluminum silicate and lacking a high temperature binder so that said silicates are not bonded together and the buffer portion is porous and relatively weak in compressive strength and (c) an outer portion having at least one layer formed from a mixture of at least 40% by weight of zirconium silicate that are coarser than the refractory particles of the prime layer, relatively fine zirconium silicate particles, and about 2 to 12% by weight of sodium fluoride as the high temperature binder which bonds the refactory particles together.

3. The mold of claim 2 in which the refractory material of the prime layer of the inner portion is stabilized cubic zirconium oxide.

4. A fired refractory mold with a total wall thickness of less than one-half inch for casting molten metals comprising (a) an inner portion comprising about 20 to 60 percent of its thickness of at least one inner prime layer formed from relatively fine refractory materials bonded together with a high temperature binder and at least one concrete layer stronger than and overlaying said prime layer so as to reinforce and strengthen said prime layer, said concrete layer being formed from refractory materials that are at least in part coarser than refractory materials of the prime layer, and said inner portion being about 20 to 40 percent of the total thickness of the mold, (b) an intermediate buffer portion comprising at least one layer of refractory material particles which are unbound and relatively low in compressive strength and (c) an outer portion comprising at least one layer formed from a mixture of at least 40% by weight of relatively coarse refractory particles coarser than refractory particles of the prime layer, relatively fine refractory particles, and a high temperature binder which bonds the refractory particles together.

5. A fired refractory mold for casting molten metals comprising (a) an inner portion comprising about 20 to 60 percent of its thickness of at least one inner prime layer formed from relatively fine refractory materials bonded together with a high temperature binder and being free of metal contaminating materials and at least one concrete layer stronger than and overlaying said prime layer so as to reinforce and strengthen said prime layer, said concrete layer being formed from refractory materials coarser at least in part than refractory materials of the prime layer and at least 5% of Which are high silica glass, (b) an intermediate buffer portion com- [prising at least one layer which is porous and has refractory particles not bonded together, (c) an outer portion comprising at least one layer formed from a mixture of 40 to percent by weight of relatively coarse refractory particles, of 35 to mesh and 10 to 60 percent of refractory particles of 150 to 400 mesh and about 2 to 12 percent of the refractory particle mixture of a high temperature binder consisting of an inorganic fluoride compound which bonds said refractory particles together, said refractory particles of 35 to 150 mesh being coarser than the refractory particles of said prime layer.

6. A fired refractory mold for casting molten metals comprising (a) an inner portion comprising about 20 to 60 percent of its thickness of an inner prime layer formed from relatively fine refractory materials bonded together with a high temperature binder and being free of metal contaminating materials and a concrete layer overlaying said prime layer which is stronger than said prime layer so as to reinforce and strengthen said prime layer, said concrete layer being formed from refractory material at least 5% of which is high silica glass and at least a part of said refractory particles being coarser than those of the prime layer, (b) an intermediate buffer portion comprising a layer of refractory particles which are not bonded together and which layer is porous and relatively low in compressive strength and (c) an outer portion comprising a layer formed from a mixture of 40 to 90 percent by weight of relatively coarse refractory particles of 35 to 150 mesh and 10 to 60 percent of refractory particles of 150 to 400 mesh, said refractory particles being at least in part coarser than those of the prime layer, said refractory particles being bonded together by about 2 to 12 percent based on the weight of said particles of a high temperature binder consisting of sodium fluoride.

7. A fired refractory mold for casting molten metals comprising (a) an inner portion comprising about 20 to 60 percent of its thickness of at least one inner prime layer formed from relatively fine refractory materials bonded together with a high temperature binder and being free of metal contaminating materials and at least one concrete layer overlaying said prime layer which is stronger than said prime layer so as to reinforce and strengthen said prime layer, said concrete layer being formed from refractory materials at least part of which are coarser than those of the prime layer, (b) an intermediate buifer portion comprising at least one layer which is relatively low in compressive strength and has refractory particles which are not bonded together and (c) an outer portion comprising at least one layer formed from a mixture of about 40 to 90 percent of relatively coarse refractory particles of 35 to 150 mesh and 10 to 60 percent of refractory particles of 150 to 400 mesh and 4 to 8 percent of a high temperature binder consisting of sodium fluoride which bonds the refractory particles together.

8. A fired refractory mold for casting molten metals comprising (a) an inner portion comprising about 20 to 60 percent of its thickness of at least one inner prime layer formed from zirconium silicate of a mesh size of about lOO to 400 bonded together with a high temperature binder and at least one concrete layer overlaying said prime layer and stronger than the prime layer so as to reinforce and strengthen said prime layer, said concrete layer formed from a major part of a zirconium silicate of a mesh size of about 150 to 400 and a minor part of aluminum silicate of a mesh size of about 10 to 150 so that at least part of the refractory materials of the concrete layer are coarser than those of the prime, (b) an intermediate bufier portion having at least one layer formed from refractory materials comprising a major part of zirconium silicate of a mesh size of about 150 to 400, a minor part of aluminum silicate of a mesh size of 10 to 150 and lacking a high temperature binder so as to be not bonded together, said intermediate buffer portion being weak in compressive strength, and (c) an outer portion having at least one layer formed from refractory materials which are at least in part coarser than those of the prime layer, said refractory materials comprising a mixture of at least 40% by weight of zirconium silicate particles having a mesh size of about 35 to 150, zirconium silicate particles of a mesh size of 150 to 400, and about 2 to 12 percent by weight of the refractory mixture of sodium fluoride to react with said zirconium silicate particles and bind them together solidly at high temperatures.

References Cited in the file of this patent UNITED STATES PATENTS FOREIGN PATENTS Great Britain Apr. 1,

OTHER REFERENCES Steel, December 28, 1953, page 88. 

